1. Field of the Invention
The present invention relates generally to a process for selectively removing an insulation film formed on the surface of a silicon wafer, a polycrystalline silicon film, and amorphous silicon (simply referred to as "silicon layer surface" hereinafter), and more particularly, to a process for selectively removing a silicon nitride film (SiNx) from a silicon layer surface on which a silicon oxide film is also present. The process of the present invention has applicability to the process of manufacturing a semiconductor device in general, and, for example, is applicable to the removal of a SiNx film at the final step of the LOCOS Process (Localized Oxidization of Silicon) in manufacturing a dynamic memory.
2. Description of the Related Art
A silicon nitride film is formed on a silicon wafer in the LOCOS process when making a dynamic memory. The LOCOS process is utilized in the manufacture of a dynamic memory for forming a boron diffusion layer used as a channel stopper and a thick silicon oxide film in an element isolation region by selective oxidation using a silicon nitride film.
In the first step of the LOCOS process, a thin silicon oxide film is formed on the surface of a silicon wafer. A silicon nitride film is grown on the surface of the thin silicon oxide film by the thermal decomposition reaction of ammonia (NH.sub.3) and monosilane (SiH.sub.4). A patterned photoresist film is formed on the nitride film by lithography so as to cover only the region where the nitride film is to remain. The nitride film is etched by a plasma etching apparatus using carbon tetrafluoride (CF.sub.4) gas. Then, boron is ion-implanted to form the channel stopper in an element isolation region using the resist film and the nitride film as a mask.
After removing the resist film from the nitride film, a thermal silicon oxide film is selectively formed (LOCOS oxidation) using the nitride film as a mask. A thin oxide film is formed on the nitride film during LOCOS. This thin oxide film is removed by wet etching. The silicon nitride film is then selectively removed from the silicon wafer. This completes the LOCOS process.
The selective removal of silicon nitride film in the last step of the LOCOS process conventionally employs the wet etching method disclosed in Japanese Patent Laying-Open No. 2-96334 and Japanese Patent Laying-Open No. 61-168925, for example.
In the conventional method, a phosphoric acid aqueous solution of high concentration (approximately 85%) is used as the etchant. The phosphoric acid aqueous solution is stored in an etching bath and heated to a high temperature (140.degree.-180.degree. C.) A silicon wafer is immersed in the phosphoric acid aqueous solution at that high temperature. Thus, the removal of a SiNx film from a silicon wafer has been carried out by a batch in the conventional wet etching method, as described above.
However, the process according to the conventional wet etching method includes the following problems:
(1) Phosphoric acid aqueous solution of high concentration is heated to a high temperature near the boiling point for use as an etchant in the conventional method. Handling a phosphoric acid aqueous solution at a high temperature is very dangerous. PA1 (2) Implementation of the method requires a circulation filtration system and the like, which complicates the structure of the apparatus. PA1 (3) Because the phosphoric acid aqueous solution is of high concentration and high temperature, equipment such as pumps and filters or pipes will be degraded in a relatively short time period, even though materials superior in corrosion-resistance and durability such as fluororesin and polypropylene are used. PA1 (4) Water evaporates from the phosphoric acid aqueous solution since its temperature is maintained in the vicinity of the boiling point during the etching process. This increases its concentration as time elapses. In order to maintain the concentration at a constant level, pure water corresponding to the amount of evaporated water must be supplied. However, pure water added in drops into phosphoric acid aqueous solution of high concentration causes water to boil locally. This necessitates various measures in the method of supplying pure water in order to avoid this danger. The structure of the apparatus will become complicated, and the manufacture thereof is not easy. It is also hard to control the amount of water to be supplied, and the controllability of the etching process is not satisfactory. Moreover, usage of the wet etching method increases the amount of consumed etchant.